A fuel cell structured by stacking a plurality of cells has been known. The fuel cell receives supplies of oxygen and hydrogen and generates electricity through electrochemical reaction. The deficient supply of hydrogen to the fuel cell during warm-up operation in a subzero environment accordingly degrades the power generation performance of the fuel cell. The excessive dryness of an electrolyte membrane in the fuel cell during high-temperature operation also degrades the power generation performance of the fuel cell.
A technique described in Patent Literature 1 given below has been known to detect an abnormality of the fuel cell, for example, deficiency of hydrogen. The technique described in Patent Literature 1 provides a cell which has a voltage change in response to an abnormality at the higher sensitivity than ordinary cells or, in other words, which degrades the power generation performance earlier than the ordinary cells (hereinafter called monitor cell) in the fuel cell and monitors the voltage of this monitor cell, so as to detect an abnormality of the fuel cell.